It is convenient to provide a variety of solids in a particulate state. However, when the particle size is small, dust problems may arise. One way of avoiding dust problems is to agglomerate the small particles into a larger agglomerated product. In addition, agglomeration of particles may confer new or changed properties, such as increased solubility or wettability.
EP 1 227 732 B1 (Niro) suggests agglomerating particles in a spray dryer having an integrated fluid bed in the bottom and a flexible filter element in the top of the drying chamber. Optimal agglomeration is obtained along the inner surface of a part of the conical bottom section of the spray drying chamber, thereby substantially reducing the amount of fine particles.
U.S. Pat. No. 5,695,701 (Niro) discloses an apparatus for the preparation of an agglomerated material from a liquid. The apparatus has a trough-shaped fluidizing path in the bottom and a nozzle for spraying an atomized liquid on the fluidized powder. Perforations in a perforated plate are arranged such that the fluidizing gas is directed i.a. in a direction perpendicular to the longitudinal direction of the trough-shaped path. Above the perforated plate, a row of filters may be arranged through which filters the leaving gas flow passes in such a way that the fine particles entrained therewith are separated and fall back into the fluidized layer.
U.S. Pat. No. 5,044,093 discloses a spray drying granulation apparatus having a fluid bed in the bottom of the spray drying chamber. In the perforated plate of the fluid bed part of the apparatus, a binder supply nozzle may be provided. The nozzle ejects atomized binder liquid vertically upwards and the droplets collide with the particles in the fluid bed. The binder supply nozzle is spraying into a so-called Wurster tube, separating the spraying zone from the remainder of the fluid bed. The growing particles are circulated in the fluid bed between the zones on each side of the Wurster tube wall until a sufficient size has been obtained.
U.S. Pat. No. 5,615,493 (Niro) describes a spray drying device comprising a fluid bed at the bottom of the drying chamber. The fluid bed is divided into a central back-mix section and a surrounding annular plug-flow section shielded from descending spray-dried particles. In the plug-flow section, nozzles may be arranged for spraying a suitable liquid on the particles in view of obtaining a further agglomeration thereof. The plug-flow along the inner circumference of the fluid bed is attained due to the constant feeding of the annular section with particles from the central section and the corresponding withdrawal of dried agglomerated particles through e.g. an overflow slot.
In the prior art discussed above an agglomeration may be obtained of the particles. However, the treatment of the particles in the prior art apparatuses and the atomizing of agglomeration fluid are not sufficiently well-defined for a uniform agglomeration to take place. The object of the present invention is to provide an apparatus which may perform a more uniform and controlled agglomeration process. Especially, it is desired to avoid or reduce the amount of fine particles in the final product to eliminate dust problems during handling of the agglomerates.
The present invention provides an agglomeration apparatus comprising
a fluid bed having a perforated plate and means for producing an upwardly directed fluidisation gas flow through the perforations of the plate for maintaining a fluidised particle layer above the plate,
a source of descending particles to be agglomerated in the fluid bed,
a nozzle for atomizing an agglomeration fluid, said nozzle being positioned below the surface of the fluidised particle layer and pointing in an upward direction, and
an outlet for discharging the agglomerated particles,
wherein the fluid bed is provided with an outer zone receiving the majority of descending particles and an inner zone connected to the outlet.
The descending particles are primarily received by an outer zone and guided towards the outlet in an inner zone. It may be ensured by a variety of ways that a majority of the descending particles are received by the outer zone. In one embodiment, the outer zone is allocated an active fluidisation area of the fluid bed greater than 50%, preferably greater than 70%. In another embodiment, a conical wall is provided above the fluid bed. The conical wall concentrates the descending particles and the particles are delivered at the inner perimeter of the fluid bed. The descending particles are subsequently agglomerated in a controlled fashion in the outer zone. The outer zone is preferably at least in a part of the flow path delimited by the wall of the fluid bed. In the outer zone a plug-flow is suitably maintained to obtain a uniform treatment of the descending particles.
The means establishing different fluidisation gas flow velocities in the zones are generally adjusted such that the zone closest to the outlet is provided with a higher fluidisation gas velocity compared to the preceding zone. The higher fluidisation gas flow velocity in the inner zone ensures that the growing particles are maintained in a fluidised state and that minor particles are blown away. The presence of an atomizing nozzle capable of atomizing an agglomeration fluid in the uniformly conveyed, fluidised layer provides for an equal wetting of the fluidised particles and, in consequence, an optimal agglomeration. The outer zone may be provided with two or more sections in which the fluidisation gas flow velocity is altered.
The sections with different fluidisation gas velocities may be provided in the bottom of the fluid bed by a variety of methods. In a certain embodiment, the plenum below the perforated plate is designed to direct more fluidisation gas to a certain section. In another embodiment, the perforations of the perforated plate are larger in the section having the highest gas flow velocity and, conversely, smaller in a section having a lower gas flow velocity. In a still further embodiment, the different zones are supplied with fluidisation gas from separate plenums.
In an aspect of the invention, the fluid bed comprises at least one partition wall provided in the fluidised particle layer. The at least one partition wall generally separates the inner and the outer zones. The partition wall may be arranged in the entire vertical extent of the fluidised layer or only in a part thereof. In a preferred aspect, the partition wall abuts the perforated plate of the fluid bed and extends essentially vertically upwards.
The partition wall may be provided to define a flow path of the particles. The shape of the partition wall depends, among other things, on the inner form of the fluid bed. In a preferred aspect of the invention the flow path of the fluidised particles in the outer zone follows the inner circumference of the fluid bed wall. In the event that the inner walls of the fluid bed are essentially a vertical cylinder, the partition wall may be a cylinder part placed co-axially in the fluid bed. Thus, in a preferred embodiment, an at least partly cylindrical partition wall is provided coaxially in the fluid bed, thereby defining a plug-flow path for the fluidised particles between the interior circumference of the fluid bed and the partition wall.
The defined flow path of the agglomerated particles ends in an outlet, which is connected to the inner zone. Suitably, the outlet is arranged via the bottom of the fluid bed. The outlet is generally provided with means for controlled particle removal, such as an overflow slot. In a certain aspect of the invention, the outlet for the agglomerated particles is positioned substantially in the centre of the fluid bed. The position of the outlet in the fluid bed centre allows for an easy adaption of the invention to existing spray dryers. Furthermore, the flow path of the particles starting at the circumference and ending in the centre of the fluid bed uses the drying capacity in an optimal way.
The source of descending particles is not limited to particles prepared by a particular technology. The descending particles may be substantially dry or partly dried. In a preferred aspect of the invention, the descending particles contain an amount of moisture, which allows them to agglomerate to some degree before reaching the fluidised layer. In a preferred aspect of the invention, the source of descending particles is selected from the group comprising a spray dryer, a filter unit, and a cyclone. In case of using a cyclone, shielding means to separate the cyclone and the fluid bed are provided. Generally, a spray dryer is preferred as the source of descending particles. In the lower part of the drying chamber, the walls are generally conical and meet the cylindrical walls of the fluid bed. Thus, in a preferred aspect of the invention, the fluid bed is provided in the lower part of a spray drying chamber, said fluid bed receiving the descending, partly dried, particles from the spray dryer. The particles are believed to slide down the conical walls of the drying chamber, thereby being concentrated in the outer zone.
In a certain aspect of the invention a spray dryer and a filter unit is combined, such that the spray drying chamber comprises internally arranged filters. Preferably, the filters are flexible filter bags mounted in the ceiling of the spray drying chamber and operated intermediately by a counter blow to release particles from the filter. The particles descend from the filters and are mixed with the partly dried particles from the spray dryer in an agglomeration area of the drying chamber. The partly dried and partly agglomerated particles continue to descend into the fluid bed.
In a flow path of the fluidised particles, one or more nozzles for atomizing an agglomeration fluid are provided, said nozzles being positioned below the surface of the fluidised particle layer and pointing in an upward direction. Nozzles pointing upward include nozzles pointing in an essentially vertical direction as well as nozzles pointing until 90 degrees off-axis relative to the vertical direction. If only a single nozzle is used, it is generally placed in the outer zone. The inner zone then serves to dry the agglomerated particles. If desired, more than a single nozzle may be provided in the outer zone.
To obtain larger agglomerates, generally two or more agglomeration nozzles are arranged along the plug-flow path of the fluidised particles. The nozzles may be evenly distributed throughout the flow path, i.e. provided in each zone or may be concentrated in some sections. In the event that a concentration of nozzles appears, it is generally preferred to concentrate the nozzles in the first part of the outer zone and/or mid part of the flow path.
The nozzle may be adjustable in height. In some applications of the present apparatus, the nozzles are in the same level as the perforated plate. However, it may be appropriate to place the nozzle either below or above the level of the perforated plate. In the event that more than a single nozzle is provided, the nozzles may be provided in different heights.
According to a certain embodiment of the invention, the agglomeration nozzle may further be provided with a tube arranged coaxially around the nozzle for supplying a secondary fluidization gas around the atomized fluid. At the tip of the nozzle, the secondary gas may be liberated through an orifice to secure the proper velocity to enhance the interaction between the particles in the fluidised layer and the atomized agglomeration liquid. It may be an advantage in the supply line for the secondary fluidisation gas to provide swirling means to increase the efficiency. To prepare a versatile apparatus suitable for different products to be agglomerated, the agglomeration nozzle and/or the tube arranged co-axially around the agglomeration nozzle may be adjustable in height.
The invention also provides a method for producing agglomerated particles, the method comprising the steps of:
maintaining in a fluid bed a fluidised particle layer above a plate having perforations through which fluidisation gas flows, said fluid bed being divided into an inner and an outer zone.
supplying the fluidised particle layer with descending particles to be agglomerated in the fluid bed,
atomizing an agglomeration liquid in the fluidised layer of particles,
guiding the fluidised particles in a plug-flow toward an outlet,
wherein the outer zone receives the majority of the descending particles and the inner zone is connected to an outlet.
The controlled plug-flow is an advantage. The uniform treatment of the particles has surprisingly resulted in an improved bulk density of the final product. Based on experiments with prior art technology in the field, it was expected that the density of the product would decrease considerably during the agglomeration process. However, in a specific sample, the bulk density was maintained at the same value. In a preferred aspect of the invention, the fluidisation gas flow velocity of the inner zone connected to the outlet is higher than the fluidisation gas flow velocity in the outer zone. The increased fluidisation gas flow velocities along the path of the fluidised particles accounts for an increased probability of the fine particles to agglomerate. The result is a final product containing only a minor or negligible fraction of small particles.
In general it is preferred in the outer zone to let the fluidised particles be conveyed along the circumference of the fluid bed walls. To obtain a higher degree of agglomeration, the sections provided with different fluidisation gas flow velocities are provided with at least one atomisation nozzle in each section. In the event that the nozzle for ejecting the agglomeration fluid is a two-fluid nozzle, the nozzle is provided with a supply of nozzle gas. Further, a secondary fluidization gas may preferably be added around the nozzle to increase the velocity around the same with the purpose of improving the interaction between the particles and the agglomeration fluid. The high velocity creates a local under-pressure securing a sufficient particle movement. Such secondary gas typically has a velocity of 5 to 50 m/s. The supply line for secondary fluidization gas may include means, e.g. such as vanes, for creating a swirl on the gas flow to increase the effect.
The difference in fluidisation gas flow velocities in the various zones or sections may be regulated to obtain the desired flow properties. In a certain embodiment, the passage from one zone or section to another is continued without any sharp borderline. However, generally it is desired to control the flow by stepwise increasing the gas flow velocity along the path of the fluidised particles. In a certain aspect of the invention, the fluidisation gas velocity in the inner zone proximal to the outlet is at least 5% higher than the adjacent zone or section in the fluid bed. The gas flow velocity is generally in the range 0.3 to 3.0 m/s, preferably 0.5 to 1.5 m/s, and a preceding zone or section may have gas flow velocity which is about 5% to about 40% lower. In a specific example, the gas flow velocity in the zone closest to the outlet is about 1.0 m/s and the gas flow velocity in the preceding zone is about 0.9 m/s.
Depending on the desired result, the descending particles may be agglomerated in various degrees. In a certain aspect of the invention, the mean particle size of the discharged particles is at least doubled compared to the incoming descending particles. Also, the particle size distribution is in general modified by the present invention because the lightest particles in a certain zone are blown higher into the drying chamber. In the area above the fluid bed, the lighter particles may agglomerate when interacting with partly dried particles from the spray dryer. Thus, the particle size distribution is generally narrower compared to a process in which fluid bed back-mix agglomeration technology is used.
An advantage of the agglomeration method according to the invention is that a more safe process may be used and at the same time the other advantages mentioned may be achieved. Without having the fluid bed agglomeration by means of supplying an agglomeration fluid to the particles, prior art agglomeration in a spray dryer demanded that a product with a higher moist content descended into the fluid bed to secure the agglomeration. The particles with high moisture content are more prone to form lumps and the process is conducted closer to dryer limits. A traditional FSD™ spray dryer may e.g. have 3-5% moist in the fluidized particles, whereas, in a FSD-GRANULATOR™ according to the method of the invention, the same product may have 1-4% moist.
In a certain embodiment, the bulk density of the discharged particles is substantially the same as the bulk density of the incoming descending particles, i.e. the bulk density after agglomeration shows substantially the same value (measured in g/ml) as if no nozzles were used. Using prior art methods, the agglomerates are nearly always of a lower bulk density than the bulk density of the descending particles. Therefore, the present method is more versatile than the previous available methods.
The agglomeration fluid may have any suitable composition. E.g., the agglomeration fluid may be water or may have essentially the same dry matter composition as the incoming descending particles. Also, the agglomeration fluid may be a melt.